The present invention relates to a method of making a part made of a reinforced plastics material by overmolding a sheet, to a device for implementing the method, and to an assembly and to a production line including said device.
It is already known that single-layer sheets can be used to make parts made of plastics materials and having a certain stiffness, in particular for making structural parts of motor vehicles.
For example, with the material known as stampable reinforced thermoplastic (SRT), which is polypropylene reinforced with glass fibers, the method consists in heating sheets of SRT, then in depositing them in an open mold, and then in closing the mold to cause the plastics material to flow and to fill the cavity of the mold.
It is also known that multi-layer materials can be used that are obtained by superposing sheets of SRT and sheets of Twintex (a trademark of Verotex), Twintex being a woven sheet of commingled glass fibers and polypropylene fibers.
The Twintex sheets become soft during the heating prior to placing the lay-up of superposed sheets in the mold, but the SRT sheets are stiff enough to enable the heated material to be handled.
In contrast, when one or more sheets of Twintex only are to be overmolded, it is difficult to place them in the mold because the heated Twintex sheets are too soft and therefore difficult to handle.
As a result, the only known methods of overmolding sheets of Twintex consist in conveying the sheets horizontally from a heating station to the mold, which mold is mounted on a vertical press so that the portion of it that is to receive the sheet(s) of Twintex is horizontal.
This difficulty of handling heated sheets of Twintex therefore imposes, as a first limit to their being overmolded, the obligation of using molds that are mounted on vertical presses.
In addition, conveying the sheet of Twintex does not enable it to be positioned accurately enough in the mold. As a result, it is preferred to leave a safety margin by cutting the sheet of Twintex larger than necessary so as to make sure that it extends over the entire desired area of the part to be made, even if the edges of the sheet of Twintex that project beyond the periphery of the part then need to be cut off after the part has been molded.
A second limit to overmolding sheets of Twintex lies in the fact that the resulting parts must be trimmed after they have been molded.